Changes in the Cetacean Assemblage of a Coastal Upwelling Ecosystem During El Niño 1997-98 and La Niña 1999

ABSTRACT

We report
results of ecosystem studies in Monterey Bay, California, during the summer
upwelling periods, 1996-99, including impacts of El Niño 1997-98 and La
Niña 1999. Random-systematic line-transect surveys of marine mammals were
conducted monthly from August to November 1996, and from May to November
1997-99. CTDs and zooplankton net tows were conducted opportunistically,
and at 10 predetermined locations. Hydroacoustic backscatter was measured
continously while underway to estimate prevalence of zooplankton, with
emphasis on euphausiids, a key trophic link between primary production
and higher trophic level consumers.

The occurrences
of several of the California Current's most common cetaceans varied among
years. The assemblage of odontocetes became more diverse during the El
Niño with a temporary influx of warm-water species. Densities of cold-temperate
Dall's porpoise, Phocoenoides dalli, were greatest before the onset
of El Niño, whereas warm-temperate common dolphins, Delphinus spp.,
were present only during the warm-water period associated with El Niño.
Rorqual densities decreased in August 1997 as euphausiid backscatter was
reduced. In 1998, as euphausiid backscatter slowly increased, rorqual
densities increased sharply to the greatest observed values. Euphausiid
backscatter further increased in 1999, whereas rorqual densities were
similar to those observed during 1998. We hypothesize that a dramatic
reduction in zooplankton biomass offshore during El Niño 1997-98 led to
the concentration of rorquals in the remaining productive coastal upwelling
areas, including Monterey Bay. These patterns exemplify short-term responses
of cetaceans to large-scale changes in oceanic conditions.